In the prior art, with a machine tool such as a machining center or the like, a workpiece to be supplied for machine processing is loaded onto the machine tool in the fixed state to a work pallet. In order to position and fix this work pallet on the table of the machine tool with good accuracy, a plurality of clamp mechanisms are equipped that are capable of positioning the work pallet on the table.
With the clamp device for a pallet of Patent Document #1, female side taper bushes 34 in which tapered holes 25 or concave portions 32 are formed are fixed at four spots upon the lower surface of a work pallet 20. And to the table side there are provided four sets of clamp mechanisms 30, 31, which engage to the four female side taper bushes 34 and clamp to them by ball lock mechanisms. Two of the sets of female side taper bush 34 and clamp mechanism 30, which are in a diagonal relationship across the center of the pallet 20, are able to position and clamp the work pallet 20 in the vertical direction and in the horizontal direction. And the other two sets of female side taper bush 34 and clamp mechanism 31 are capable of clamping the work pallet 20 in the vertical direction.
With the clamp device having a datum function of Patent Document #2, in order to position and fix a work pallet 3, four sets of clamp mechanism 4a, 5a, 6a, and 6a are provided to a base member 2. And four engagement ring members 4b, 5b, 6b, and 6b are provided to the lower portion of the work pallet 3, corresponding to the clamp mechanisms 4a, 5a, 6a, and 6a. It is arranged to be possible to clamp the work pallet by positioning in the vertical direction and in the horizontal direction with the two sets of clamp mechanisms 4a, 5a and the two engagement ring members 4b, 5b that are in a diagonal relationship, and it is arranged to be possible to clamp the work pallet 3 with the other two sets of clamp mechanisms 6a, 6a and the two engagement ring members 6b, 6b. 
With the devices described in these Patent Documents #1 and #2, since at least four clamp mechanisms and two sets of positioning mechanism in the vertical and the horizontal direction are provided, accordingly the overall number of components that make up such a device for positioning and fixing a work pallet is large, and also the structure is complicated, the space required for installation is great, and the manufacturing cost is high.
Moreover, since this structure supports and fixes the work pallet at four locations, accordingly it is difficult to support the central portion of the work pallet, and elastic deformation of the work pallet easily takes place.
And, in the case of a compact work pallet, a difficulty is also encountered from the point of view of ensuring sufficient space for installation of the four clamp mechanisms.
With the work holding system described in Patent Document #3, there are provided a base side reception unit 1 and an adapter 74 fixed to the work pallet. An annular male polygon member 25 is fixed by bolts to an outer circumferential portion of the reception unit 1 than a central stepped portion thereof. A tapered outer circumferential surface that reduces upwards is formed on this male polygon member 25, and this tapered outer circumferential surface is formed in a triangular shape as seen in plan view.
A female polygon member 87 having a tapered inner circumferential surface capable of engaging with the tapered outer circumferential surface of the male polygon member 25 is fixed by bolts to the adapter 74, and moreover a wear ring member 90 is fixed thereto by bolts so as to contact against the upper end surface of the male polygon member 25. The work pallet is lowered from above the reception unit 1, so that the tapered inner circumferential surface of the female polygon member 87 on the work pallet side engages with the tapered outer circumferential surface of the male polygon member 25, and the wear ring member 90 is contacted against the upper end surface of the male polygon member 25. thereby, not only is the work pallet positioned in the vertical direction and in the horizontal direction, but also relative rotation around the vertical axis is restrained, and it is fixed to the reception unit 1 by a ball lock mechanism provided to the reception unit 1 that includes an air cylinder and a compression spring.
Furthermore, with the tool assembly described in Patent Document #4, a structure is provided in which a tapered projection 15 whose cross section is non-circular is formed on the base end portion of a tool 12, a tapered concave portion 25 into which the tapered projection 15 can fit is formed on a holder 11, and the tapered projection 15 is fitted into the tapered concave portion 25 and is fixed thereinto by a bolt.
Patent Document #1: Japanese Laid-Open Patent Publication H07-314270
Patent Document #2: Japanese Laid-Open Patent Publication 2001-38564
Patent Document #3: U.S. Pat. No. 5,370,378 Publication
Patent Document #4: U.S. Pat. No. 2,536,899 Publication